1. Field of the Invention
The present invention relates to a semiconductor laser diode, and more particularly, to an improved semiconductor laser diode including a clad layer having an asymmetric refractive index.
2. Description of the Related Art
Generally, a semiconductor laser diode has a comparatively small size and a threshold current for laser oscillation, which is less than a threshold current of a general laser device. Therefore, semiconductor laser diodes are widely used as an element for high-speed data recording and reading in a communication field or players in which optical disks are used.
FIG. 1A is a cross-sectional view of a conventional semiconductor laser diode. FIG. 1B illustrates a refractive index profile and optical field distribution of the semiconductor laser diode of FIG. 1A.
Referring to FIGS. 1A and 1B, a conventional semiconductor laser diode has a configuration in which an n-clad layer 11, an n-waveguide layer 12, an active layer 30, a p-waveguide layer 22, and p-clad layers 21 are sequentially deposited on a substrate 10. An etch stop layer to form a ridge is interposed between the p-clad layers 21. A p-electrode layer 40 is formed on the top surface of the p-clad layer 21, and an n-electrode layer 50 is formed on the bottom surface of the substrate 10.
Here, the p-clad layer 21 formed above the active layer 30 and the n-clad layer 11 formed below the active layer 30 have lower refractive indexes nupper and nlower, respectively than the refractive index nactive of the active layer 30, and the refractive index nupper of the p-clad layer 21 is identical with the refractive index nlower of the n-clad layer 11.
However, in a conventional optical storage device, a semiconductor laser diode is required to have a smaller far field vertical beam divergence angle than approximately 19°. A semiconductor laser diode having a configuration as described above cannot obtain a small far field vertical beam divergence angle if a thin waveguide layer is formed to obtain a large output of power.
A refractive index profile of a semiconductor laser diode including a clad layer having an asymmetric refractive index to solve the problem described above is illustrated in FIG. 2A. Referring to FIG. 2A, a p-clad layer and an n-clad layer are formed on both sides of an active layer respectively, and a refractive index of the n-clad layer is higher than a refractive index of the p-clad layer. Accordingly, the p-clad layer and the n-clad layer have asymmetric refractive indexes with the active layer as a center.
A semiconductor laser diode having the configuration as described above has an advantage to decrease a far field vertical beam divergence angle by dispersing a near field. However, asymmetry of the near field increases significantly which leads to deflection of a laser beam according to an output power as illustrated in FIG. 2B, and potential barrier energy, which is inversely proportional to a refractive index, is reduced in an n-clad layer, thereby suffering more carrier overflow.